Tar-derived pitches have long been employed as binder materials in the production of several carbonaceous materials, such as graphite electrodes, or as precursors for carbon fibers. Although the figures will vary somewhat depending upon the particular product in which the pitch is to be incorporated, in general commercially useful pitches which are to be employed as binder materials should desirably exhibit softening point temperatures in the range of about 100.degree.-200.degree. C. and coking values of about 50-60 weight percent. See, for example, S. Ragan and H. Marsh "Science and Technology of Graphite Manufacture", Journal of Materials Science, Vol. 18, pp. 3161-3170 (1983). Moreover, as is discussed in British Pat. No. 2,045,798 pitches which are to be employed as carbon fiber precursors preferably possess a softening point of 150.degree.-250.degree. C.
As is employed herein the term "softening point temperature" refers to the temperature at which the viscosity of the pitch is reduced to the degree required by the Mettler Softening Point Method of ASTM D 3104-75. The term "coking value" refers to the coke-forming properties of the pitch as measured by the Modified Conractson Test described in C. Karr, Jr., Analytical Methods for Coal and Coal Products, Academic Press, Vol. II, p. 521 (1978), ASTM D-189.
In the past, the preparation of pitches exhibiting such desirable properties has generally entailed distillation of the precursor petroleum and coal tars in order to remove the lighter components from these substances, by processes such as that disclosed is U.S. Pat. No. 3,140,248. However, the final pitch yields of such distillation processes are typically relatively low, e.g. about 10-30 weight percent, based upon the weight of the tar starting material. Other processes, such as that disclosed in U.S. Pat. No. 4,096,056 which involves heating the tar precursors while blowing an oxygen containing gas into the reactor, will produce somewhat higher yields but require relatively high temperatures of more than 300.degree. C. and are therefore economically undesirable. Thus it would be desirable to possess a process which would produce high yields of pitch exhibiting useful properties from tar precursors, which process would operate at lower temperatures so as to be more energy efficient.
It is also known in the art that the addition of nitro-functional group-containing aromatic compounds into pitches will increase the coking rate of such pitches. For example, German Pat. No. 1,015,377 shows the addition of aromatic nitro compounds, such as nitroamines, nitrophenols and the like to pitch, while S. Yamada, Kogyo Kapaku Zashi, Chemical Abstracts 58, 4343h, 4344a, b, c, d show the addition of dinitronaphthalene and dinitrobenzene to pitch in order to increase its coking value. It should be noted that pitches are composed primarily of aromatic hydrocarbons and are generally not known to possess functional groups. As stated by A. Hoiberg, Bituminous Materials, Vol. III, p. 158, Interscience (1958), "It will be seen then that the compounds positively identified as pitch constituents consist predominantly of condensed polynuclear aromatic hydrocarbons or heterocyclic compounds containing from three to six rings. Methyl and hydroxyl are the only substituent groups encountered and, in a relatively few cases, partial or total saturation of one ring is found." Thus while pitches may contain some small percentage of nitrogen, such nitrogen is generally not present in the form of nitro groups bound to the aromatic rings of which the pitch is composed.
Thus, the above described processes require the additional step of adding and mixing the nitro-substituted aromatic additive to the pitch. Moreover, such nitro-functional group containing aromatic compounds are relatively expensive. It would therefore be desirable to possess a pitch having nitro-functional groups bound to the aromatic constituents of the pitch itself in order to avoid such addition and mixing steps.
British patent application No. 2,045,798 shows a process for preparing a pitch from a tar, which process comprises mixing the tar with a nitrating agent selected from nitric acid, an organic nitrate and nitrogen pentoxide and heating the resulting mixture at a temperature of 150.degree. to 400.degree. C. It is noteworthy however that this patent specifically states that (1) nitro-functional groups and/or nitric ester-functional groups are almost undetected or cannot be detected in the resulting pitch and (2) it is impossible to obtain a pitch having a high softening point (i.e, 150.degree. C. to 250.degree. C.) if the heating temperature during pitch formation is lower than 150.degree. C.
Accordingly, it is an object of this invention to provide a pitch having nitro-functional groups bound to the aromatic component of said pitch.
It is a further object of this invention to provide a pitch having nitro-functional groups bound to its aromatic constituent, which pitch additionally possesses a commercially desirable softening point temperature and coking value.
It is another object of this invention to provide a process for producing pitch from tar in increased yields.
It is yet another object of this invention to provide a process for producing pitch from tar at relatively low temperature and at a relative high rate so as to be more energy efficient. The foregoing and additional objects will become apparent from the following description and examples and the attached figure.